It is not only the total absorption capacity of absorbent products, such as diapers, sanitary napkins and incontinence protectors, that is of interest, in other words the maximum volume of liquid that a product can absorb, but also the ability of the product to transport liquid quickly away from the liquid receiving surface and into the product. The liquid accommodating properties of an absorbent product are normally determined by ascertaining the acquisition time of said product, by which is meant the time taken for a given volume of liquid to penetrate into the product.
A typical method of determining the acquisition time of a product, is to place a pipe or tube on top of the product with the tube orifice pressed thereagainst, and then fill the tube with a determined quantity of liquid. This method is very imprecise in many instances, since the accuracy of the process is influenced by the liquid accommodating properties of the product. The process of liquid accommodation results in the formation of liquid columns of greater or smaller heights, depending on the speed at which the liquid is accommodated, these liquid columns exerting pressure on the product and therewith influencing the result of the measuring process. This makes comparison between the liquid accommodating properties of different products difficult. In order to illustrate the effect of the liquid column, it can be mentioned that the acquisition time in respect of one and the same product has been measured at 8 mm liquid column and 13 mm liquid column and has been found to have a duration of 39 and 26 seconds respectively. The height of the liquid column can thus have a great bearing on the result of the measuring process. This is a serious drawback, since the height of the liquid pillar generated when using this method is dependent on the liquid accommodating properties of the product to be tested. Acquisition times determined in accordance with this method cannot be readily compared with each other and merely provide a rough gauge of the differences in liquid accommodating properties between products.
In "the test mess--part IV", James P. Hanson, NONWOVENS WORLD WINTER 1998, pp 54-57, there is described a method with which the influence of the liquid column is reduced, by providing the measuring tube with a runoff pipe of given height. The amount of liquid delivered can be determined by supplying liquid at a constant rate of flow and measuring the amount of liquid that runs off through said pipe. This enables the acquisition time to be determined. The method also gives an understanding of the volume of liquid received by the product per unit of time, after the liquid in the delivery pipe has reached the runoff level, although it is unable to provide information relating to the liquid accommodation properties of the product prior to this point in time. There is consequently the need of a method that will enable a study to be made of the development of liquid accommodation from the beginning of the process to the point at which an intended volume of liquid has been absorbed. The present invention addresses this need.